1. Field of the Invention
The present invention relates to a PWM converter system having a function to provide the sine wave power supply current whose power supply power factor is nearly 1 to supply electric power from the power source side to the load side or regenerate electric power from the load side to the power source side.
2. Related Background Art
A sine wave PWM converter has been popular as a power converter that greatly reduces harmonic currents generated by a power converter. The PWM converter is connected to an input side AC (Alternating Current) power source via an AC reactor or a transformer, and a smoothing capacitor and a load are connected between DC (Direct Current) terminals located on the converter""s output side. Therefore, the PWM converter is controlled so that the sine wave power supply current (i.e. AC reactor current) which has the same phase as the power supply voltage is provided during the loading state when electric power is supplied from the power source side to the load side.
On the other hand, as seen in a solar power generating device, when electric power is regenerated from the battery side (i.e. DC power source side) to the AC power source side, a converter is controlled so that the sine wave power supply current whose phase is reverse to that of the power supply voltage is provided. Specifically, a power supply current amplitude command, which is a current command whose phase is the same as that of a power supply voltage, is provided to make the DC voltage of a smoothing capacitor become constant and thus the PWM converter is controlled so that the detected value of the power supply current agrees with the command.
As described above, a current sensor is required to detect an input alternating current of a converter. For example, as disclosed in Japanese Application Patent Laid-Open Publication No. 07-213067, at least two current sensors are required when a three-phase converter is used and one current sensor is necessary when a single-phase converter is used. Further, if a positive or negative arm switching element of a converter short-circuits, a voltage of a smoothing capacitor also short-circuits causing an overcurrent, which results in damaging the switching element. Therefore, in order to protect the switching element, a direct current sensor is placed between the output side of the converter and the smoothing capacitor to detect overcurrent and block gate signals.
Furthermore, as the relevant art, Japanese Application Patent Laid-Open Publication No. 06-153526 has disclosed a method of detecting a three-phase alternating current from a direct current of a three-phase inverter so as to make the AC side current sensor of the three-phase inverter unnecessary.
In a conventional PWM converter system, two current sensors are required on the converter""s alternating current input side and one on the direct current output side when a three-phase PWM converter is used, and one current sensor is required on the input side and one on the output side when a single-phase PWM converter is used.
Further, the art disclosed in Japanese Application Patent Laid-Open Publication No. 06-153526 requires a high-speed A/D converter or a microprocessor.
The main object of the present invention is to provide a PWM converter system which controls a converter without requiring the current sensor at the input side of the converter and performs the control equivalent to a converter system with the current sensor, resulting in reducing the overall cost and the size of the converter system.
A PWM converter system in accordance with the present invention comprises current detecting means which detects an input current of a converter from a direct current at the output side of the converter and the gate signal of the converter, and a computing unit which controls a power supply current of the converter based on the detected current. Accordingly, the PWM converter system controls the converter without requiring the current sensor at the input side of the converter and performs the control equivalent to a converter system with the current sensor attached to it.
A single-phase PWM converter system in accordance with the present invention, which is connected to a single-phase AC power source, employs a sample-and-hold circuit, as current detecting means, which samples and holds a direct current in the state wherein one of two-phase gate signals of a positive or negative side arm is ON and the other phase is OFF; determines the sign of an output of the sample-and-hold circuit according to the two-phase gate condition; and detects an input current of a converter. Furthermore, the PWM converter system employs means for controlling an input voltage of a converter so that the detected current value agrees with a current command. This enables the PWM converter system to perform the control equivalent to a PWM converter system with the current sensor attached to it. Moreover, the signal waveform output from the sample-and-hold circuit has a period of 180 degrees of a converter""s input-side AC voltage and continuously changes in synchronization with a power supply current.
Further, a PWM converter system in accordance with the present invention blocks all gate signals of a converter when an output of the sample-and-hold circuit exceeds a predetermined overcurrent level. Accordingly, when an overcurrent occurs due to overload or some other reasons, the PWM converter system can be protected quickly.
Next, a current detecting part of a three-phase PWM converter system in accordance with the present invention, which is connected to a three-phase AC power source, comprises a first sample-and-hold circuit which samples and holds a direct current in the state wherein one of three phases of a positive or negative side arm switching element of the three-phase PWM converter system is ON and the other two phases are OFF, and a second sample-and-hold circuit which samples and holds a direct current in the state wherein two phases are ON and the other phase is OFF.
Further, a three-phase PWM converter system in accordance with the present invention employs means for calculating an effective current Iq and a ineffective current Id of a power supply current based on the output of said two sample-and-hold circuits and the detected value of the power supply voltage phase. This makes it possible to control an alternating current input voltage of the converter in response to the calculated values of Iq and Id. Consequently, it is possible to perform the control of active and ineffective currents without using the current sensor at the input side of the converter, which is equivalent to the control by a similar PWM converter system with the sensor.
Furthermore, a three-phase PWM converter system in accordance with the present invention blocks all gate signals of the converter when at least one output from a first or second sample-and-hold circuit exceeds a predetermined overcurrent level. As a consequence, the converter system can quickly be protected from overcurrent due to overload or some other reasons.